This invention relates to battery chargers and more particularly, it relates to circuits for supplying a charging potential from a power source to a battery. Still more particularly, the invention relates to a circuit for supplying a charging potential from a vehicle electrical system to a separately carried battery, such as a sealed lead-acid battery in a towed vehicle, i.e., trailer, or the battery power pack of a portable cellular phone, etc.
It is highly desirable, and indeed a requirement, that certain towed vehicles, i.e., trailers of a certain size rating, have a system for actuating the towed vehicle brakes should the towed vehicle break away from a towing vehicle. These towed vehicles typically include electric brake-actuation components powered from the towing vehicle electrical system during normal towing conditions. However, power for energizing the electric brake-actuation components is not available from the towing vehicle when the towed vehicle separates from the towing vehicle.
To provide a power source on the towed vehicle for energizing the brakes if a breakaway occurs, a battery is provided on that vehicle. In order for the battery to apply sufficient power to the towed vehicle brakes to stop the towed vehicle and hold it for a prescribed time, the battery must be fully charged while the vehicle is being towed. For example, a fully charged battery will hold the towed vehicle brakes engaged for at least fifteen minutes for purposes of stopping the towed vehicle and preventing rolling once the towed vehicle stops. However, prior to being towed, the vehicle may remain inactive for an extended time period, during which the battery charge may be significantly or completely depleted. Furthermore, as noted hereinafter, the steady-state or "float" level of a lead-acid battery is well below its fully charged level in any event.
Accordingly, it is desirable to charge the towed vehicle battery from the towing vehicle electrical system while the vehicle is being towed. Prior art systems connect the battery of the towed vehicle to the electrical system of the towing vehicle through a diode, or a series connection of a diode and a resistor. Thus, a voltage drop, at least equal to the forward voltage drop of the diode, exists between the tow vehicle electrical system and the towed vehicle battery. This voltage drop slows the rate at which the battery charges and in most cases prevents the towed vehicle battery from being fully charged. A depleted battery charged from a low supply voltage may take hours, or even days, to charge to the level of the applied voltage. During that time, should the trailer disconnect from the towing vehicle, the battery will not have sufficient capacity to adequately actuate the towed vehicle brakes.
Known circuits for charging a battery from a vehicle electrical system have a number of other disadvantages and limitations. In particular, they apply a voltage to the battery which is directly proportional to the supply voltage of the vehicle electrical system and, accordingly, they are unable to effectively control the voltage applied to the battery. For example, the optimum charging voltage for the battery is dependent upon the ambient temperature. However, prior art circuits for charging a battery from a vehicle are unable to compensate for ambient temperature variations. Thus, prior art circuits are unable to charge the battery to a higher voltage at low ambient temperatures than at a higher ambient temperatures. Additionally, such prior art circuits are unable to supply a relatively high charging potential until the battery is fully charged and a lower "floating potential" once the battery reaches the charging voltage. On the one hand, if the optimum charging voltage is applied to the battery after it is fully charged, the electrolyte will boil away. On the other hand, if a smaller voltage is used to charge the battery, a significant time period is required to fully charge the battery.
Chargers are known for charging a battery from a stable power supply such as an AC wall outlet. These chargers use either a step-up converter or a step-down converter, depending upon the relationship between the supply source potential and the battery charging potential. The supply voltage must be greater than the charging voltage for a step-down converter and less than the charging voltage for a step-up converter for the charger to supply an appropriate voltage to the battery. However, the supply voltage from a vehicle, e.g., a car or truck, will vary substantially, depending upon whether the car engine is running or off, the length of time that the engine has been off, etc. The voltage on the battery being charged will also vary widely depending upon how long the battery being charged has been idle. Accordingly, the voltage output by a vehicle electrical system may be larger or smaller than the voltage on the battery being charged at any given time. Thus, step-up and step-down voltage converters are unable to reliably provide an adequate battery charging voltage from a vehicle power supply.